In many industries, thin materials are used for construction. Many times these materials are used in materials, or oriented very close together with other thin materials. In the aerospace industry, often times a first material, usually a hot material, requires holes for cooling purposes, while a second material, typically a cold material, does not have holes so as to ensure the structural integrity of the second material.
One such situation where this arises is in liners for hot sections of gas turbine engines, such as an augmenter liner, for example. Laser drilling allows for these holes to be created in the first material. However, once the laser drills through the first material, the second material may be inadvertently struck by the laser, called a backstrike. This may be caused by variability in controls for laser positioners, the number of laser shots used, laser output controls, and material deficits such as thickness or voids in castings. This backstrike is undesirable and may cause damage to the second material, such as creating a crack in the second material.
In some materials this strike is detectable due to a color change in the material visible on the opposite side or by visual inspection between the thin materials being drilled. However, many times the materials are impossible to inspect visually due to geometric or material constraints and do not change colors when struck. Thus, damage caused by the backstrike may be completely undetectable except by destroying the assembly, such as by cutting up the materials to examine them.
Therefore, a non-invasive method of detecting backstrikes during the process of drilling closely-spaced, thin materials is needed.